Traffic signal control is a key element in traffic management that affects the efficiency of urban traffic systems. Most major cities currently employ adaptive traffic signal control systems where the traffic light timing is adjusted based on the current traffic situation. Examples of such adaptive traffic signal control systems are SCATS (Sydney Coordinated Adaptive Traffic System) and SCOOT (Split Cycle Offset Optimisation Technique).
Control variables in traffic signal control systems typically include phase, cycle length, split plan and offset. A phase specifies a combination of one or more traffic movements simultaneously receiving the right-of-way during a signal interval. Cycle length is the time required for one complete cycle of signal intervals. A split plan defines the percentage of the cycle length allocated to each of the phases during a signal cycle. Offset is used in coordinated traffic control systems to reduce frequent stops at a sequence of junctions. SCATS appears to attempt to equalize the degree of saturation (DS), i.e., the ratio of effectively used green time to the total green time, for all the approaches. SCATS appears to employ a heuristic approach to compute cycle length, with various parameters that have to be tuned to achieve this objective. In addition, all the possible split plans have to be pre-specified and a voting scheme has to be used in order to select a split plan in order to obtain approximately equal DS for all the approaches.
Systems and control theory has also been recently applied to traffic signal control. Optimization-based approaches have also been considered. However, one of the major drawbacks of these approaches is the issue of scalability. In other words, such approaches do not scale well with the size of the road network while ensuring satisfactory performance.
Backpressure routing is a technique that has been mainly applied to communication networks, where a packet may arrive at any node in the network and can only leave the system when it reaches its destination node. However, backpressure routing cannot be simply implemented for traffic signal control. For example, backpressure routing requires the knowledge of the destination of each packet and treats packets with different destinations differently. In traffic signal control, however, vehicles traveling in the same direction through a junction cannot be differentiated based on their destination and controlled differently. As a result, implementing backpressure routing in traffic signal control requires the assumption that all the vehicles have a common destination, which is not reasonable. Secondly, backpressure routing assumes that the controller has complete control over routing of the traffic around the network. In traffic signal control, the controller does not have control over the route picked by each driver. Thirdly, backpressure routing also assumes that the network controller has control over the rate of sending each commodity data during each time slot. However, the traffic signal controller does not have control over the flow rate of each traffic movement once a phase is activated.
A need therefore exists to provide a traffic signal control method and traffic signal controller that seek to address at least one of the abovementioned problems.